This invention relates to new and useful improvements in rotatable augers used in the manufacture of hollow core concrete slabs.
These slabs are formed from a relatively dry concrete mix in a machine moveable along a fixed casting bed by the extruding action of the slab from the machine onto the fixed bed.
Conventionally, such machines utilize a plurality of rotating auger assemblies mounted within an open ended enclosure with a hopper receiving the concrete mix feeding by gravity into one end of the enclosure, being compressed and compacted by the rotating augers in a mould chamber, passing over a trowelling member and then being deposited rearwardly of the machine in the form of a smooth surfaced multi-apertured concrete slab normally having pre-stressed or post-stressed reinforcing wires or cables extending longitudinally therethrough.
Conventionally, such augers of existing technologies are structured so that the diameter of the external core around which the auger flight is situated, constantly increases towards the downstream end of the auger to a maximum diameter at or near the point where the auger flight ceases. This allows the combined forces of compression and compaction to be exerted on the relatively dry concrete material. Compression is caused by the increasing of the diameter of the outer core of the auger in the form of a tapered cylinder, moving concrete material into a smaller and smaller area within the mold chamber of the machine.
Compaction is effected by a high frequency vibrator imparting energy throughout the entire surface of the auger and imparting an increasingly greater effect on the concrete material as it approaches the downstream end of the auger. In addition, further vibration may be provided to the upper or hammer plate of the mould chamber by an exterior vibrator directly or indirectly acting on the hammer plate.
The combination of these two forces, compaction and compression, is essential to the moulding of the concrete material into its final consolidated form and each of these forces acts in concert with the other but has an increasing effect on the material as it proceeds along and over the downstream part of the auger.
These conventional augers have an outside core which has a constant increase in the diameter of the external core at an angle of approximately 10.degree.-15.degree. to the longitudinal axis of the core, reaching the maximum at the downstream portion of the auger where the auger flights runs out, it being understood that the external diameter of the auger flighting remains substantially constant. The principal difficulty with conventional auger constructions is the difficulty in obtaining the guaranteed bond of the reinforcing strands or cables, with the concrete. This is because the conventional auger with a gradually increasing taper to the outer diameter of the core leads towards the production of a negative flow of the concrete thus contradicting the free concrete flow required in order to provide the necessary bond, due to the slope of the core surface between flights being rearwardly or opposite to the relative movement between the concrete and the core.